Patient electrode isolation

ABSTRACT

A system for preventing the inadvertent electrocution of a surgical patient through accidental coupling of probes and common equipment to ground. Isolation of the patient from ground is obtained through the use of a light emitting diode and a photo transistor in the probe circuit.

United 7 States Patent Holsinger [54] PATIENT ELECTRODE ISOLATION [72]Inventor: William Perry I-lolsinger, Vienna,

The United States of America as represented by the Secretary of theDepartment of Health, Education, and Welfare Filed: Nov. 19, 1970 Appl.No.: 90,969

Assignee:

[56] References Cited UNITED STATES PATENTS 9/1970 Schuler ..330/30D8/1969 Gilbert ..330/207P [451 Oct. 17, 1972 OTHER PUBLICATIONS IEEETransactions Bio Medical Electronics, Vol. 17,

Primary Examiner-J. D. Miller Assistant Examiner-Harry E. Moose, Jr.Attorney-Browdy and Neimark [57] ABSTRACT A system for preventing theinadvertent electrocution of a surgical patient through accidentalcoupling of probes and common equipment to ground. Isolation of thepatient from ground is obtained through the use of a light emittingdiode and a photo transistor in the probe circuit.

1 Claim, 1 Drawing Figure DIFFERENTIAL sounce AMPLIFIER FOLLOWERSPASSIVE 2? 24 l0 1 I 25 $4. N I!) J D e 3/ 8 ,m E MINUS 0 TO 5 OP AMPSLu ,2 [34 35 PLUS (I OP AMPS EXTER I 32 CIRCUI 37 h OUTPUT 4 EXTERNALCIRCUITRY 4s I OUTPUT 4 PATENTEDHBIUIQR f 3.699.389

' DIFFERENTIAL SOURCE AMPLIFIER FOLLOWERS v ELECTRODES PLUS 'ro WW 0PAMPS I INVENTOR. WILL /AM 1? HOL swam ATTORNEYS PATIENT ELECTRODEISOLATION BACKGROUND AND SUMMARY .The present invention relates toisolation circuits and, more particularly, to an isolation circuit withan optical arrangement for decoupling an electrical surgical probe fromground.

There are only two ways that a patient can accidentally be electrocutedwhen connected to electrodes while undergoing an operation or adiagnostic examination. The first ofthese is through interelectrodeleakage or short circuits, where the monitoring device leaks lethal"currents'between the electrodes. The second is through coupling withother monitoring equipment, common equipment grounding being the mostcommon culprit. Both possibilities must 'be eliminated, however, toguarantee patient safety.

The hazard of interelectrode leakage can be eliminated byseries-connecting a passive current limiting network,i.e.,one thatintroduces no current of itself, between the electrodes and themonitoring device.

The second cause of accidental electrocution is the most probable.Whenever a patient is grounded, he is vulnerable to almost everyconceivable interaction between equipments, such as ground loopcurrents, electromagnetically coupled transients, and even poor hospitalgrounding techniques. It takes as little as microamperes to set theheart muscle into fibrillation. This could still occur even where usingthe aforementioned technique since the ground connection to the patientcannot be current limited. However, if the whole input amplificationnetwork were floating and battery powered, this major cause would alsobe eliminated. FM modulation and demodulation techniques suffer fromcritical frequency controls and bulky component needs, and still do notguard against coupled electromagnetic transients through the transformerwindings or leakage currents through transfor- BRIEF DESCRIPTION OF THEDRAWING The drawing shows a schematic diagram of the isolation circuitryDETAILED DESCRIPTION Referring now to the lone FIGURE of the drawing,which sets outa schematic diagram of the invention, there are providedinput terminals 10, l1 and 12, these inputs furnishing signals fromother surgical test or diagnostic devices (not shown) in use during anoperation, such as intracavitary transducers, electronic probes and thelike. Connected to terminal 10 there is a resistance 13, while a similarresistance 14 is tied to terminal l2; terminal 11, on the other hand isconnected with a junction point 20.

A pair of oppositely poled diodes l5 and 16 are tied between the far endof resistance 13 and junction point 20, while another pair of oppositelypoled diodes 17 and 18 are located between the far end of resistance 14'and junction 20. The output signal from resistance 13 is applied to aseries connected resistance 21 from whence it is fed to an FET sourcefollower amplifier 22. From amplifier 22 the signal is impressed onseries resistance 24 and then to a differential amplifier 25, this'amplifier having a feedback loop to its'input and including aresistance26.

A parallel channel, similar to the one described above, includes aseries resistance 27 connected to the output terminal of resistance 14,the output of resistance 27 being applied to an FET source followeramplifier 28. From amplifier 28 the signal goes through mers leakagecapacitances. OPtical isolation ex-- periences no such drawbacks andfurther allows wider frequency bandwidths using practical components.

Thus, the present invention offers many improvements and advancementsover the weaknesses and drawbacks of prior systems. The dual-functionisolation technique disclosed herein is inexpensive, compact, reliable,and even adaptable to existing equipment. If all patient monitoringequipment were isolated in this manner, accidental electrocution couldnever occur.

It is, accordingly, an object of the present invention to overcome thedefects of the prior art, such as indicated above.

It is another object of the present invention to provide a safeenvironment to prevent electrocution.

It is yet another object of the present invention to provide a devicefor isolating a patient, particularly a series resistance 30 beforeacting as a second input to differential amplifier 25. A lone resistance31 is tied between the output end of resistance 30 and junction point20. The output of amplifier 25 is applied as on input to a driveramplifier and drives 32 after passing through a series resistance 33.Feedback loop 36 provides the necessary gain control while the otherinput to amplifier 32 is connected to junction point 20.

Potential for the isolation network is supplied by two batteries 34 and35; the positive terminal of battery 35 and the negative terminal ofbattery 34 connecting to junction point 20.

The driver signal, as produced by amplifier 32, passes through adropping resistance 37 before being applied to an optical coupler 38,shown generally within the dotted lines. The return from coupler 38 isconnected to the negative terminal of batter 35.

The optical coupler 38 consists of a light emitting diode 41 and aphototransistor 42. Positive potential is supplied to thephototransistor by means of a lead 43 connected to one electrode, whilenegative potential is furnished by lead 44 and resistance 45 connectedto the output electrode of the phototransistor.

The output of the optical coupler, and therefore of the isolationnetwork is furnished by an output terminal 46, connected to the outputelectrode of the phototransistor 42, this output converted from theinfrared radiation generated by diode 41 acting as a variable resistanceto generate a proportional current for subsequent amplification.

In operation the isolation network receives input signals on inputtenninals 10, 11 and 12, as from intracavitary transducers, and othersurgical instruments,

fore applyingthem to monitoring equipment via output terminal 46.interelectrode leakage, or short circuits, where the monitoring deviceleaks lethal current between electrodes is eliminated by seriesconnecting a passive current limiting network, or one that introduces nocurrent of itself, between the electrodes and the 'monitoring devices,and including such passive network as a portion of the present overallisolation network. Resistances 21 and 27 limit any current from theamplifiers 22 and 28 to a low enough value so as not to exceed therating of diodes 1s, 16, 17 and 18, even if full battery supply voltageswere short-circuited. Two diode pairs 15, 16, 17 and 18, reverseconnected, can handle either positive or negative voltages. Since thejunction of the diodes can never exceed i0.7 volts (silicon), 0.7 voltsthrough resistances l3 and 14 limit interelectrode current to 3.5microamperes, which is sufficiently low for safe connection even tointracavitary transducers. The high 200K input resistors 13 and 14necessitate amplifiers 22 and 28 to have very high input impedances (FETsource follower) with extremely low bias currents; otherwise theamplifiers could not track the in put waveforms. Diodes 15, l6, l7 and18 must be fast acting so as to dissipate the transient energy arisingfrom a defibrillator pulse. Here; the 200K input resistors 13 and 14limit the high voltage current.

The elimination of accidental coupling between monitoring and otherequipment, by forming a common ground, is accomplished by the circuitryof the source follower amplifiers 22' and 28, differential amplifier 25,driver amplifier 32 and optical coupler 38. Optical isolationexperiences none of the drawbacks of electromagnetic coupling orinteraction between components since electrical current flow is brokenby the 'beam of light. An operational amplifier 32 modulates the lightemitting diode 41 and is biased sufficiently far in the linear operatingregion to preserve signal fidelity. Thus the emitted infrared radiationis optically coupled to a phototransistor 42 which acts as a variableresistor to generate a proportional current at its output terminal 46for subsequent amplification and use by monitoring equipment.

From the above description of the structure and operation of theinvention, it is obvious that the present device offers a number ofimprovements over the drawbacks and shortcomings of prior art isolationcircuits. The device discloses a dual function isolation technique thatis inexpensive, compact, reliable and adaptable to existing present dayequipment. If all patient monitoring equipment were isolated in thismanner, accidental electrocution could never occur.

Obviously many modifications and variations of the present invention arepossible in light of the above teachings. It is therefore to beunderstood that within the scope of the appended claims the inventionmay be practiced otherwise than as specifically described.

What is claimed is: i

1. A patient electrode isolation circuit for preventing accidentalelectrocution of a patient comprising first, second, and third inputmeans for receiving 1 signals from electrical equipment as might be usedin the treatment of a patient;

a first passive current limiting network connected between the first andsecond'input means, said network bemg a resistance and a diode, saidresistance preventing possible high voltage on the first and secondinput means from breaking down the diode;

a second passive current limiting network connected between the secondand'third inputmeans, said networkbeing a resistance and a diode,'saidresistance preventing possible high voltage on the second and thirdinput means from breaking down the diode;

a first source follower series connected in the first input means;

a second source follower series connected in the third input means;

a third passive current limiting network connected between the input ofthe first source follower and the second input means, said network beinga resistance and a diode, said diode being poled oppositely from thediode in the first passive current limiting network and the resistancepreventing any excessive interelectrode leakage in the first sourcefollower from breaking down the diode;

a fourth passive current limiting network connected between the input ofthe second source follower and the second input means, said networkbeing a resistance and a diode, said diode being poled oppositely fromthe diode in the second passive cur- I rent limiting network and theresistance preventing any excessive interelectrode leakage in the secondsource follower from breaking down the diode;

a differential amplifier connected to receive the outputs of the firstand second source followers;

a driver amplifier connected to receive the output of the differentialamplifier;

an optical coupler consisting of a diode and a phototransistor connectedto the output of the driver; and i output means connected to thephototransistor.

1. A patient electrode isolation circuit for preventing accidentalelectrocution of a patient comprising first, second, and third inputmeans for receiving signals from electrical equipment as might be usedin the treatment of a patient; a first passive current limiting networkconnected between the first and second input means, said network being aresistance and a diode, said resistance preventing possible high voltageon the first and second input means from breaking down the diode; asecond passive current limiting network connected between the second andthird input means, said network being a resistance and a diode, saidresistance preventing possible high voltage on the second and thirdinput means from breaking down the diode; a first source follower seriesconnected in the first input means; a second source follower seriesconnected in the third input means; a third passive current limitingnetwork connected between the input of the first source follower and thesecond input means, said network being a resistance and a diode, saiddiode being poled oppositely from the diode in the first passive currentlimiting network and the resistance preventing any excessiveinterelectrode leakage in the first source follower from breaking downthe diode; a fourth passive current limiting network connected betweenthe input of the second source follower and the second input means, saidnetwork being a resistance and a diode, said diode being poledoppositely from the diode in the second passive current limiting networkand the resistance preventing any excessive interelectrode leakage inthe second source follower from breaking down the diode; a differentialamplifier connected to receive the outputs of the first and secondsource followers; a driver amplifier connected to receive the output ofthe differential amplifier; an optical coupler consisting of a diode anda phototransistor connected to the output of the driver; and outputmeans connected to the phototransistor.